The new things you need to develop new features are far and far between
99% of web software written these days could fulfil identical use cases on an IBM 3270 from 40 years ago. You enter something into a form and it gets stored in a database. You enter something into a field and it generates a report. That's all Amazon, Facebook, Google, any e-commerce site are.
Sure it might be nice to use a new version of that new JS framework that all the twitterati are going crazy about, but does it deliver value to the business that justifies the risk and investment?
And yet none of those things did arise 40 years ago. All of the nuances of all the code written since then make a difference, despite duplicating "identical use cases".
If it were only that, we would have an easy time. The new things you need to develop new features are far and far between.